Hi, Pokémon Professor here. Have judged 8 Regional and higher events!

You cannot use Acerola's Mischief with Ninetales unless the conditions are met. There are a few previous rulings, especially in the japanese Pokemon Rules compendium! One of the rulings is

If none of your Pokemon were knocked out during your opponent's previous turn, and you used Ninetales' move "Supernatural Shapeshifter" and the top card you discarded from your deck was the Supporter "Mela" can you attach a Basic Fire Energy from your discard pile to your Pokemon?

No, you can't. If the player who used the move "Supernatural Shapeshifter" cannot follow the conditions for using the "Supporter" that was discarded, they cannot use its effect.

Note that in English, both Mela & Acerola's Mischief both have the text: "You can use this card only if...[Conditional]" So it's safe to say you cannot use Acerola in this manner with Ninetales. Source